What is the bonus scene?

According to the game manual,

In Expert mode, the distaff does not glow in response to musical threads (except the ones you spin yourself). Also, there is no musical staff to help you identify the threads you hear. You literally have to play the game by ear! Note: Players who dare to experience Loom in Expert mode are rewarded with a bonus animated scene near the end of the story. This scene does not appear in any other mode.

I completed the game on expert, but did not notice the bonus scene.

What is the bonus scene?

Solution 1:

When Bobby is being held prisoner by the Clerics, and the guard catches him trying to investigate the scrying orb, the guard offers him a deal. He will let him look at the orb, if Bobby lets him look at his face without the robe.

"Legends say it is death to look upon a Weaver uncloaked. Naturally, us clerics pay little heed to such foolish rumours. Still, I'm curious. I'll let you look into this sphere if you let me lift your hood. Deal?"

In the original version, the cut-scene cuts forward. The Cleric is gone, and Bobby comments "Can't say I didn't warn him". It is left up to the player to interpret what happened.

When playing on expert mode, you see the cleric lift Bobby's hood. As soon as the hood is lifted, Bobby's head emits a blinding light. The Cleric screams, as his body is reduced to its skeleton, and he is sucked into the blinding light that is Bobby's head.

Solution 2:

Possibly you skipped the bonus scene, or you didn't realize that it was a bonus scene.

You can always rewatch it from youtube: